Control system for velocity modulation tubes



April 13, 1943- c. v. Ll-TTON.

CONTROL SYSTEM FOR VELOCITY MODULATION TUBES .Ffiled Aug. 27, 1940 INVENTOR. CHA/FLES H U7 7011/ ATTORNEY.

Patented Apr. 13, 1943 uNirEo STATES CONTROL SYSTEM FOR VELOCITY MODULATION TUBES Charles V. Litton, Redwood City, Calif assignor t to International Standard Electric Corporation, v New York, N. Y., a corporation of Delaware Application August 27, 1940, Serial No. 354,359

2 Claims.

This invention relates to velocity modulation vacuum tubes and more particularly to arrange ments for producing an efiicient control system for modifying the velocity of electrons in a beam.

It has been proposed to alter the velocity of electrons in a beam by an alternating field through which the beam passes, to allow the electrons so modified to traverse a distance so as to become bunched or grouped and to extract energy from the grouped electrons by means of a resonant circuit.

With such a system if, in general, the'opening through which the beam passes is a large fraction of a wavelength, and the beam extends over this area, the acceleration given some electrons near the outer part of the beam maybe very different from that applied to the electrons'near the center of the beam. Consequently the accelerations may be such as to produce poor bunching or grouping with accompanying loss in efficiency.

It is an object of my invention controllably to dimension 'the area or shape of the electron 'beam so as to produce eflicient operation of a tube of the velocity-modulation type.

This may be accomplished by keeping the controlled area of the beam down to a width not exceeding about of a wavelength. The beam maybe solid and 20% or less of a wavelength in diameter or may be made in the form of a hollow annulus the width of which does not exceed the 20% limitation. V

A better understanding of my invention, as well as the objects and features thereof, may be had from the following particular description thereof made in connection with the accompanying drawing, in which Fig. 1 shows a conventional type of velocity modulation tube incorporating features of my invention.

Fig. 2 shows a modified form of velocity modu lation tube according to my invention, and

Figs. 3 and 4 are curves used in explaining the principles of my invention.

In Fig. 1 is shown one form of velocity modulation tube. This tube comprises an electron emitting cathode ill from which electrons are accelerated in a beam either by potential applied to the tube casing or by other accelerating electrodes. This beam passes then through a pair of grids l2, I3, associated with a resonant chamber ll. This chamber is made resonant to a desired frequency so that electrons of said beam are alternately accelerated and decelerated as they pass between grids I2 and I3. These velocity modified electrons then pass through a drift tube l4 wherein they become bunched or grouped due to their different velocities and are then passed between two grids 16, ll associated with a second resonant chamber l5. Resonant chamber I5- operatesto extract energy from the grouped electrons tending to slow down the entire stream. The electrons then pass on to a target electrode [8 wherein the remaining energy is dissipated in heat.

The tube may be operated as an oscillator'by providing a coupling loop [9, as shown in Fig. 1, for intercoupling chambers l5 and H to produce a feedback of energy and may be provided'with using the generated high obvious that the potential across the grid opening of [2, l3, or 16, ll, will not be ntirely uniform but will follow the shape of a sine wave or a portion thereof. It is clear then that if the openings of these grids are large, for example, a half wavelength of the curve 30, as shown in Fig. 3,

the electrons of the beam passing between the grids will not be uniformly accelerated, but will be accelerated to a considerably greater extent in the center than at the edges. Accordingly, the electrons upon reaching output grids l6, ll, will not be uniformly bunched but will be bunched more highly at the center providing thedrift tube is dimensioned for this maximum effect. Accordingly, a tube with such a large opening would no operate with very high efficiency.

This is particularly true when it is considered that as a rule the electrons will not travel in 'a straight path but will be interspersed so that upon reaching the extracting grids l6, l1 some of the slower moving electrons will be at the center of the stream and the bunching will be extremely non-uniform. ,This effect is not likely to cause a disturbance at the longer wavelength, since generally in such cases the openings of the grids l2, (3, will be essentially a small portion of the total wavelength. However, as shorter wavelengths are produced, the dimension of the grid may become an appreciable fraction of a wavelength. Accordingly, if even fairly efficient operation is to be maintained, this Would necessitate an extremely sharp focusing arrangement to maintain all of the electrons substantially in a straight line path along the drift tube.

This harmful efiect is greatly reduced if the grid opening, that is the opening in the resonant chamber through which the beam passes, which may contain conductive elements to secure ellective distribution of the control voltages, or in other words the maximum effective dimension of the beam is made a small fraction of a wavelength, for example, not exceeding 20 of a wavelength. This portion of the wave is indicated by the lightly shaded area 3| of Fig. 3, th grid cavity being indicated by the blank portion 32. It can be seen that if the opening is maintained at substantially this dimension then a substantially uniform acceleration of the electrons throughout the beam may be obtained because of the slight difference in potential over this area. This potential ratio may be obtained quite simply as follows:

If m: the peak voltage and n== the voltage at the edge of the opening then n may be expressed as follows:

Accordingly, it is possible to so construct the tubes or to control the size or dimension of the beam that the portions thereof acted upon at the grid is not subjected to potentials having more than substantially a 5% difference ratio.

Instead of confining the beam to a small area such as set forth in connection with Fig. 1, the beam itself may be made in the form of a hollow annulus, the radial thickness of which is sufilciently small in dimension so as to receive substantially uniform control acceleration, that is, to not have more than substantially a 5% difference in the potentials over the area of the beam. A tube structure for producing such results is shown in Fig. 2. This tube, as illustrated, is substantially similar to that shown in Fig. 1, and accordingly corresponding similar elements are given similar reference characters. However, in

this tube the cathode ID has the active electron from cathode in then proceed as a hollow beam through the control grids l2, l3 of chamber II and to the extracting grids l6, ll of chamber l5. While the tube may be used as an oscillator when provided with a feedback connection, as shown in Fig. 1, the tube of Fig. 2 is shown arranged for amplification with an input lead 22, and output lead 20.

It is clear that since the electron beam is of hollow annular form the potential operating to affect the velocity of electrons in the beam need be effective only over a limited range equal to the thickness of annulus 2|. An explanation of the operation of the tube in this respect may be made in connection with Fig. 4.

It is desirable that the electron stream be made as large as possible in order to handle larger amounts of power. By using the annular construction a relatively greater amountof power may be transmitted in the beam than with a smaller beam confined to a narrow pencil. It is clear then that if the openings or grids for controlling the beams are made relatively large, for example, in the neighborhood of a wavelength long, as shown in Fig. 4, and the cathode is made also of such dimensions, the beam then passing through the grids may be an annulus having a thickness of approximately 20% of a wavelength, so that the control potential applied on each side of the grid will be effective over the lightly shaded areas AI, 42, of Fig. 4. With such an arrangement a grid having a large area and consequently a beam containing considerable energy may be handled.

Such grids will have greater capacity therebetween than the smaller grids of Fig. 1. This may, however, be taken care of in constructing the tube so as to tune the system properly to the wavelength.

It is clear from the teaching and discussion of my invention as outlined above that I have devised a system for efiiciently operating velocity modulated tubes. Furthermore, by utilization of the annular beam system not only is the high efficiency preserved but an extreme increase in power over the other types of systems may be obtained. It is clear that either of these systems may apply equally well to oscillation generators such as shown in Fig. 1, or to amplifier tube circuits such as shown in Fig. 2. Furthermore, if desired other elements may be applied for the purpose of concentrating the beam or maintaining a parallel flow thereof.

While I have disclosed certain features of my invention in connection with the illustrations thereof, it should be distinctly understood that this is made merely by way of illustration and not as a limitation thereof. What I consider as my invention and desire to protect by Letters Patent is embodied in the accompanying claims.

What I claim is:

1. A vacuum tube structure for use in velocity modulated tubes, comprising an electron emitting cathode, means for producing a beam of electrons from said cathode, a resonator shell resonant at substantially the operating wavelength of the structure, means in said shell through which said electron beam may pass for modifying the velocity of electrons in said beam, said last named means comprising beam confining means for confining said beam to a continuously effective diametric dimension of not more than substantially one fifth of a wavelength at said operating frequency, said beam confining means including an electrode transverse to said beam and having a generally circular open portion with a radial dimension of substantialy one fifth of a wavelength at said operating frequency, whereby the voltages instantaneously effective upon said beam in any one electrode do not differ by more than five percent.

2. A vacuum tube structure for use in velocity modulated tubes, comprising an electron emitting cathode, means for producing a beam of electrons from said cathode, and resonator means for varying the velocity of electrons in said beam, said cathode including an annular emitting surface for emitting an annular beam, said annular emitting surface having a radial thickness not greater than one fifth of a wavelength and a mean efi'ective diameter equal to substantially a wavelength. CHARLES V. LITTON. 

